150 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



2d, the mode of union between the plates of the calyx — whether movable or 



immovable. 

 3d, the condition of the stem — whether the young joints were formed beneath the 



proximal ring of the calyx, or beneath the top stem joint. 



The morphological importance of these characters is shown by the fact that 

 among the earlier Crinoids they appear as well defined as among those of 

 later epochs, which indicates that the origin of the three groups dates back 

 to a much earlier time. 



The Crinoidea Inadunata represent the simplest form, their dorsal cup 

 being composed invariably of only two rings of plates, or three when infra- 

 basals are present. It has no supplementary plates, except an anal piece, 

 but this is not represented in all of them. The radials at four sides of the 

 cup are in lateral contact, and the arms are free from the radials up. 



In the structure of the ventral disk we recognize two different plans; 

 the one exhibiting the utmost simplicity, being composed exclusively of five 

 large oral plates forming a pyramid ; the other showing considerable com- 

 plexity at the posterior interradius, which is drawn out into a sac or tube. 

 Upon these two plans we divide the Inadunata into two subgroups : The 

 Ij^adunata Larviformia, and the Inadunata Fistulata, both embracing 

 monocyclic and dicyclic forms. 



The calyx of the Larviformia consists of but few plates ; viz. : basals 

 ■ — occasionally infrabasals — radials and orals, the latter forming a closed 

 pyramid which rests against the radials ; the mouth is closed, and the am- 

 bulacra are not exposed to view. The arms are simple, non-pinnulate, one 

 to each ray, and they are composed of long, quadrangular joints which, 

 except upon the radials, are united by close suture. The anus is excentric, 

 either piercing the posterior oral, or situated between the orals and radials, 

 and is sometimes extended out to the end of a long, narrow tube. One can 

 scarcely imagine a more complete parallelism than there is between these 

 ancient Crinoids and the larval state of recent ones, and it was this that 

 suggested the name of the group, which, as we beheve, represents the most 

 primitive type of the Crinoids generally. That we do not find the Larvi- 

 formia prior to the Niagara, but only subsequent to the advent of the other 

 groups, may be accounted for by the small size of their species. The calyx 

 of AUagecrinus Austinii from the Scotch Carboniferous is no larger than 

 a small grain of sand, and it is very possible that the earliest forms were 

 altogether microscopic. The total absence of supplementary plates, and the 



